This invention is related to a process for forming a composite structure of sheet molding compound (SMC) and to the resulting composite structure.
SMC, which is a composition of reinforced thermosetting polymeric material such as cured polyester resin reinforced with fiber glass, is widely used for many parts used in trucks, buses, automobiles, appliances, business equipment, recreational vehicles and recreational equipment such as water skis and the like. In the field of construction, SMC has been molded into such items as bathtubs, shower stalls, vanities and residential doors. In automobiles, SMC parts are used to replace sheet metal parts such as fenders, doors, hoods, and roofs. In some cases, the entire exterior of the auto has been made from SMC. Some of the advantages of SMC over metal parts are that SMC is free from corrosion and rapid deterioration from weathering which has always been a serious problem with metals; SMC parts are more resistant to denting than metals and SMC can be economically molded into intricate parts which can not be done with metals.
After molding SMC into a part, the surface of the SMC is porous, rough and often has other imperfections and in general is not a smooth shining class A surface that can be finished readily or painted. To obtain a smooth paintable surface of high quality, the surface of the part is usually cleaned and primed with an appropriate primer and then painted or a topcoating material applied such as a polyester gel coat. An in-mold coating process can also be used as shown in Masuda et al U.S. Pat. No. 4,557,889 issued Dec. 10, 1985 in which a coating is sprayed into a mold before the SMC part is formed. In the molding process, the coating is adhered to the surface of the part. Paint injection molding also can be used in which paint is injected under pressure into the mold containing SMC to decorate the SMC part that is formed. To reduce cost and improve efficiency, it would be desirable to eliminate any extra steps as shown above in the above conventional processes used for finishing SMC parts.
Conley et al U.S. Pat. No. 3,679,510 issued July 25, 1972, shows the lamination of a relatively thick thermoplastic sheet with a polyvinyl fluoride outer surface to a glass fiber reinforced polyester resin sheet (SMC) to form a laminate. Such laminates having a major portion of a thermoplastic sheet have a low modulus and are not useful for many automotive and truck parts that require high modulus materials for rigidity and thermal stability such as auto and truck hoods. When an attempt was made to reduce the thickness of the thermoplastic layer with the materials taught by Conley et al, the imperfections from the fiberglass in the SMC layer show through and one cannot form a part that is acceptable for automotive or truck use.
It would be desirable to form SMC parts that are relatively rigid, have a smooth surface that is free from surface imperfections of the SMC substrate and requires essentially no surface preparation prior to painting. Also, it would be desirable to form parts that have a high quality durable and weatherable finish that have the desired color that matches other painted parts of an auto or truck and that can be used in assembly without additional preparation. The novel process of this invention provides the above; it forms SMC parts that are relatively rigid and have a class A surface that can be readily painted or forms a relatively rigid SMC part that has a high quality colored surface or prefinished surface finished with a layer or layers of paint that matches adjacent painted or colored surfaces of metallic or plastic parts and can be used directly for auto or truck assembly.